A flight attendant didn't get the joke, according to Stuff.co.nz.
When she saw the T-shirt, worn by Wynand Mullins, she asked him to
remove it because the slogan was intimidating other passengers.

Mullins told the reporter the person next to him started to laugh but
the flight attendant was quite serious and she set out in search of
another shirt for him to wear. However, she mustn't have found one
because she never returned, according to the story.